Optimizing EGS stimulation diagnostics: Quantifying hydro-mechanical decoupling and fracture closure dynamics using distributed strain sensing at Utah FORGE

Accurate characterization of fracture closure dynamics is critical for stimulation optimization in Enhanced Geothermal Systems (EGS). This study quantifies hydro-mechanical decoupling during post-shut-in closure using synchronized distributed strain sensing (DSS) and pressure diagnostics in four Utah FORGE stages (4, 5, 8, and 9). Our primary objective is to determine whether mechanical closure ( T m e c h ) systematically precedes hydraulic closure ( T h y d ), to quantify decoupling magnitude and variability, and to evaluate whether HMCS can provide physically interpretable closure-stiffness signatures. Mechanical closure is identified from strain-rate decay and hydraulic closure from G-function derivative behavior with manual quality control for complex stages. All stages show T m e c h < T h y d , with decoupling factors of 1.8–3.9 and lag times of 3.4–8.9 min. We further apply a Hydro-Mechanical Closure Stiffness (HMCS) framework based on phase-wise secant stiffness, revealing stage-dependent stiffening behavior and a distinct screen-out signature in Stage 9. This work contributes a reproducible field workflow for DSS-based mechanical closure picking, integrated hydro-mechanical timing diagnostics, and practical HMCS interpretation guidance. Because the dataset is limited (n = 4), conclusions are interpreted as exploratory and mechanistic rather than statistically definitive.